Inter-individual dependency in visual cognition: Observers adjust perceptual decisions based on others’ perception

Observing how others perceive objects is a fundamental human behavior as it provides valuable information for adapting to new environments. In the present study, we demonstrate that human observers adjust their perceptual decisions based on others’ perceptions, even when those perceptions are entirely irrelevant to the ongoing behavioral goal. In Experiment 1, pairs of participants performed a visual working memory task where they remembered the orientation of a simple visual stimulus and reproduced the orientation after a short delay. A response cue, presented shortly after the stimulus, determined which participant should report the orientation (i.e., stimulus ownership) for that trial. Therefore, participants could ignore the stimulus orientation if the cue indicated that their partner should report it. The orientation reports were randomly assigned to each participant, and the orientations between trials were entirely independent. Nevertheless, one participant’s orientation reports were systematically biased by the orientation presented in the previous trial, even when their partner had reported the orientation, demonstrating that participants adjusted their perceptual decisions based on their partner’s stimulus. This was replicated in Experiment 2 where the stimulus ownership was determined at the time of stimulus presentation so that the encoding of the stimulus into working memory was completely unnecessary for one participant. The same pattern of results was obtained in two follow-up experiments where the stimuli for each participant were spatially separated (i.e., lateral presentation) and where different stimuli (i.e., orientation and location) were used for each participant in the pair. In the last experiment, we found that the reports were systematically biased by the partner’s decision even when the partner’s stimulus was invisible. Together, these results demonstrate the inter-individual dependency in visual cognition: observers automatically adjust their perceptual decisions based on how others perceive objects even when such decisions are completely irrelevant to the task at hand.